1. Field of the Invention
The present invention relates to a machine and a method for drying a fibrous web, in particular a cardboard, paper or tissue web, wherein the traveling fibrous web is acted upon by steam and hot, moist air in the region of a preceding drying zone and, following the preceding drying zone, is fed to a downstream drying zone which includes a drying cylinder, for example a Yankee cylinder, as well as a hood allocated thereto.
2. Description of the Related Art
U.S. Pat. No. 7,351,307 B2 and WO 2008/077874 describe a method for the production of a voluminous tissue web wherein a so-called belt press is used in conjunction with a hot air hood and the use of steam for dewatering the fibrous web to a certain dry content. It is particularly important for tissue machines of this type to reduce energy consumption, especially during the drying process to achieve a predetermined dry content. Moreover there is a requirement to increase the dry content with minimal energy expenditure.
A method is known, for example from EP 1 959 053, wherein exhaust air from the hood allocated to a Yankee cylinder is supplied to the hot air hood of a belt press. A large amount of steam is used by the dryer units which include a Yankee cylinder with allocated dryer hood or respectively a belt press. The energy costs associated with this result in accordingly higher costs in the paper production. Previously, the exhaust air of a hood allocated to the Yankee cylinder containing a high energy content was used to preheat the hood combustion air for the Yankee dryer and water used in the paper machine.
The technology based on the use of a belt press involves an even higher steam consumption compared to conventional tissue machines which overall, leads to a negative energy balance. For this technology which is based on a belt press, hot, moist air is required as an additional energy, for which previously exhaust air from the hood allocated to the Yankee cylinder was used. To this end it was previously necessary to mix the exhaust air from the Yankee hood with fresh air in order to reduce the temperature of the air supplied to the belt press to the value necessary in this belt press. Accordingly, the moisture and enthalpy of the hot air were reduced. The temperature of the Yankee hood exhaust air is regularly higher than the temperature at which the belt press can operate.
What is needed in the art is an improved method, as well as an improved device or machine, wherein the drying process, especially in regard to the energy requirement for dewatering of the fibrous web, is further optimized. To this end, the drying process, for example in the case of combined drying in a belt press and a subsequent drying unit including a Yankee cylinder with allocated dryer hood is to be accordingly optimized.